Whoa. He asked you about opening the relationship two years ago, and you found out that he already has someone. So he was cheating, and now expects you to teach this one and accept her?
I know you have the D/S thing going on, but respect is a must in this lady's book of life. How does he even know that you and her are romantically compatible? He just brought home a person, and you are supposed to be a-okay with it and just deal? Wow.